ovs-vsctl: Allow commands to tell ovs-vsctl to try again later.

ovs-vsctl: Allow commands to tell ovs-vsctl to try again later.

The "wait-until" command to be introduced in an upcoming commit needs to
be able to tell the ovs-vsctl main loop to try again later, since the
condition that it is looking for has not yet been satisfied.  This commit
adds the infrastructure for this.  (It's being broken out into a separate
commit because it modifies scattered code in ovs-vsctl.c and thus might
be easier to review this way.)